1. Field of the Invention
This invention relates to a stator for a DC dynamo electric machine which is composed of permanent magnets disposed around a substantially non-magnetic, columnar solid body rotor or a non-magnetic, columnar coreless solid body rotor having a closely stacked winding to provide a strong magnetic field in an extremely large air gap defined between the permanent magnets.
2. Description of the Prior Art
In known various types of DC dynamo electric machines, a magnetic circuit, via the route defined as pole pole piece-air gap-armature-air gap-pole piece is set up by forming a flux path loop of very low reluctance. The armature has a slot core or smooth core, and, in the case of the drag cup type, there is provided a stationary central flux return core.
A very small air gap defined between such an armature core or central core is commonly referred to as a working air gap. Accordingly, as to working air gaps in DC dynamo electric machines, the abovesaid very small air gap is common in the art. However, in a columnar solid body rotor previously proposed by the present applicant, a columnar dust core is affixed to the rotor shaft, a thin cylindrical insulator is disposed around the dust core, and then a rotor winding is mounted on the outer periphery of the insulator in large quantity. In a columnar solid body coreless rotor similarly proposed by the present applicant, a sleeve-like insulator is mounted around a rotor shaft, and a large amount of rotor winding is disposed between the insulator and the outer periphery of the rotor. Both of these rotors are rod-like solid body rotors and have the advantages of large winding capacity, excellent mechanical strength, low inertia and low inductance; therefore these rotors exhibit an excellent control performance, withstand a high-speed rotation and an impulse input and produce a high output.
In the columnar solid body rotor, however, a dust core whose iron loss is substantially negligible is employed, and the equivalent permeability of the rotor winding as a whole is held small. Accordingly the volume-conversion mean permeability .mu. of the rotor may be set to a small value as, for instance, 2, in some cases. Since the permeability of the armature core or central core of the prior art DC machines can be considered to be about 800, for example, the permeability .mu.=2 of the columnar solid body rotor can be regarded as substantially equal to the space or air gap permeability.
As the columnar coreless solid body rotor is non-magnetic as a whole, the space defined between field poles facing the non-magnetic or substantially non-magnetic rotor can be regarded as an extremely large air gap. Accordingly, the pole piece-to-pole piece magnetic circuit around such a solid body rotor is similar to an open magnetic path which would be formed in a case of the armature core or central core being removed in the known DC machine. With such a pole piece-to-pole piece magnetic circuit, a multipole structure is impossible. The reason is that fluxes are centered at the sides of adjacent pole pieces along paths of shorter distance than that between the centers of the pole pieces, resulting in no effective flux being supplied to the non-magnetic or substantially non-magnetic solid body rotor. As a consequence, the magnetic stator having an extremely large air gap corresponding to such a solid body rotor as mentioned above is limited specifically to the double-pole type.
For applying a strong magnetic field to the extremely large air gap, it is suitable to employ anisotropic barium ferrite, strontium ferrite or like permanent magents of high coercive force.
In the case of forming a magnetic path having an extremely large air gap by using such permanent magnets under a configurational restriction of what is called the configuration-output ratio, excessive leakage flux occurs due to the ratio between the air gap length and the length of a leakage magnetic shunt of the main magnetic path. This makes it difficult to converge field flux on the magnetic pole.
Now, consider such a double-pole structure as shown, for example, in FIG. 1, in which reference numeral 3 indicates a field yoke, 1 and 1' designate main magnets, 2 and 2' identify pole pieces and 4 denotes a columnar coreless solid body rotor. Letting .PHI..sub.1 represent the magnetic flux over the entire area of each magnetic pole with that pole area being the product of the magnetic pole with lp corresponding to the rotor and the length of a magnetic path perpendicular to the drawing sheet, .PHI..sub.2 represent the magnetic flux between the inclined portions of the magnetic poles, .PHI..sub.3 represent the magnetic flux between the end portions of the main magnets, .PHI..sub.4 (including .PHI..sub.4 ' identifying only an opposite direction) represent magnetic flux between each side of the magnetic pole pieces and the yoke, .PHI..sub.5 (including .PHI..sub.5 ' identifying only an opposite direction) represent the magnetic flux between each side of the main magnets and the yoke and .PHI..sub.m represent the overall magnetic flux of each main magnet, it follows that EQU .PHI..sub.m =.PHI..sub.1 +2(.PHI..sub.2 +.PHI..sub.3 +.PHI..sub.4 +.PHI..sub.5)
Though differing with the magnetic properties of the main magnets and the magnetic path configuration used, the field flux .PHI..sub.1 in an air gap represented in FIG. 1 as lg is as small as, for example, 25% of the overall flux .PHI..sub.m. In other words, the flux is almost consumed as leakage flux. This results in a reduced amount of effective flux, introducing many difficulties in the construction of a specific operative magnetic path.
A structure for preventing leakage magnetic field between pole pieces by applying thereto a reverse magnetic field is disclosed in U.S. Pat. No. 3,334,254 issued to Koher. A structure having a plurality of main magnets disposed in contact with pole pieces on many sides is set forth in U.S. Pat. No. 3,906,268 issued to Graffenried.
The structure by Kober is of the type rotating a multipole magnetic field and has the defect that a magnetic field of an additional magnet is set up on the side of a main magnet at an angle of nearly 90.degree. to the direction of the magnetic field of the main magnet. In the Graffenried structure, a plurality of main magnets are efficiently contacted with a many-sided pole piece. These structures are supposed to provide a magnetic circuit, formed via the route pole piece-air gap-armature (stator in the Kober patent)-air gap-pole piece which establishes a flux path of very low reluctance. If the Kober or Graffenried structures were to be applied to the aforementioned non-magnetic or substantially non-magnetic solid rotor (in the case of the Kober patent, a substantially non-magnetic stator) to which this invention is directed, flux emanating from one adjacent pole piece to the other is centered at the portion of the latter along a path of the shortest distance between them, and no effective flux is supplied to the armature (the stator in the Kober patent).
FIG. 2 is explanatory of a stator of a DC dynamo electric machine having the columnar coreless solid body rotor previously proposed by the present applicant. The illustrated structure blocks a large amount of leakage flux generated in a field magnetic path of the double-pole structure employing permanent magnets defining therebetween an extremely large air gap by applying a reverse magnetic field equivalent to the leakage field. Accordingly, the field flux is focused on the extremely large air gap to provide for a markedly increased ratio of the field flux to the overall flux of the main magnet, i.e. effective flux.
In FIG. 2, main magnets 1 and 1', which are ferrite or like permanent magnets of high coercive force, are disposed on the inside of a field yoke 3, and pole pieces 2 and 2' are respectively mounted on the main magnets 1 and 1' in opposed relation to a columnar coreless solid body rotor 4. In leakage magnetic paths which develop between the main magnets 1 and 1', blocking magnets 5 are respectively disposed. In a closed loop passing through the main magnet 1, the blocking magnet 5, the main magnet 1' and the yoke 3 along the line T-U, and letting flux of flux density designated as Bg in the air gap be positive in the direction indicated by the center arrow, letting lm and l.sub.5 represent the lengths of the main magnets 1 and 1' and the blocking magnet 5, respectively, and letting -Hm and H.sub.5 represent magnetic fields at the working points of the main magnets and the blocking magnet, respectively, the main and blocking magnets are selected such that -2Hm.multidot.lm=H.sub.5 .multidot.l.sub.5. By doing so, the leadage magnetic field between the magnetic poles is blocked by a reverse magnetic field equivalent to the leakage magnetic field. Leakage magnetic fields between each main magnet and the side of each pole piece and between the main magnet and the yoke and a leakage magnetic field between each blocking magnet 5 and the yoke 3 are similarly blocked with blocking magnets 6 and 6' by adopting the abovesaid method in connection with broken-line closed loops a-b-c and d-e-f-h. With such an arrangement, the ratio of field flux .PHI..sub.g to the overall flux .PHI..sub.m of the main magnet, that is, the effective flux could be increased up to 70% under the otherwise unfavorable condition of the extremely large air gap.